This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 100 63 287.4, filed on Dec. 19, 2000, the entire disclosure of which is incorporated herein by reference.
The invention relates to a method and an apparatus for forming and especially plastically deforming a metal sheet on a forming mold having a mold contact surface with a simple or complex curvature, with a minimized friction between the metal sheet and the mold contact surface.
Various methods and apparatus are known in the art for subjecting a metal sheet to a mechanical deformation to impose a curvature on the sheet, by means of a tensile stretching or drawing process. To carry this out, the metal sheet, which is to be deformed, is first arranged on the mold contact surface of a suitable forming mold or the like, and is then subjected to tensile stretching or drawing by suitable tensile drawing apparatus, for example by being clamped and pulled by at least one so-called xe2x80x9cclamping shoexe2x80x9d. In this context, it is necessary to lubricate the interface between the metal sheet and the mold contact surface of the forming mold, to minimize the friction between the metal sheet and the contact surface of the mold or the tensile drawing means, so as to allow the metal sheet to slide or shift on the mold surface in a compensating manner as the sheet is being drawn. This is necessary so that the intended formed contour can be smoothly and uniformly imposed on the metal sheet, without over-stretching some areas and under-stretching other areas, which would occur if friction between the metal sheet and the mold contact surface prevents smooth straining and deformation of the metal sheet.
Such lubrication of the metal sheet is typically achieved by applying a layer of oil or grease or the like between the mold contact surface and the metal sheet. Unfortunately, thereafter, it is necessary to carry out a degreasing and cleaning operation to remove the oil or grease, so as to prepare the metal sheet for subsequent coating applications or the like, or other subsequent processing steps. For example, it is often necessary to carry out an intermediate annealing of the metal sheet that has been pre-formed in the above manner, and thereafter to further deform the annealed metal sheet in a so-called calibration step in order to achieve the final deformed configuration of the metal sheet, whereby a slight spring-back of the metal sheet must be taken into account to arrive at the final configuration. Before such an intermediate annealing step, it is almost always necessary to degrease the lubricated pre-formed metal sheet.
The above described known methods, which involve several distinct steps utilizing several different processes, are generally rather complicated, time-consuming and thus costly to carry out. Especially, the required intermediate processing steps, such as a degreasing step and an intermediate annealing step, as well as the necessary post-processing steps, such as the calibration deforming step, significantly add to the complexity and cost of the conventional methods. Carrying out such methods with such distinct intermediate steps and processes also requires a substantial amount of additional handling of the metal sheet workpiece, as well as a capital investment in additional molding, heat-treating, and processing equipment for carrying out such several steps and processes.
In view of the above, it is an object of the invention to provide a method and an apparatus for plastically deforming a metal sheet without requiring additional measures or processes during or in connection with the forming process. Particularly, it is a further object of the invention to avoid the need of a separate heat treatment for improving the plasticity of the metal material of the metal sheet, and to avoid the need of lubricating the workpiece with oil or grease or the like, while still supporting the workpiece on the forming mold in a nearly friction-free manner. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved according to the invention in an apparatus for forming a metal sheet, including a stretching or drawing mold with a contoured contact surface on which the metal sheet is to be formed, and a tensile drawing machine with which the edges or side margins of the metal sheet are pulled to apply a stretching tension to the metal sheet on the drawing mold. Especially according to the invention, the drawing mold has plural air holes penetrating through the contact surface thereof. These holes are connected to a source of compressed or pressurized air, so as to introduce the pressurized air to form an air cushion between the metal sheet and the contact surface of the drawing mold.
The above objects have further been achieved according to the invention in a method of forming a metal sheet, especially using the apparatus of the invention as described herein. In this method, the metal sheet is subjected to a tensile drawing process in order to form a singly or complexly curved contour, for example a convex or concave or compound curvature, onto the metal sheet. The inventive method further includes the following steps or phases in one particular embodiment thereof.
In a preliminary step, a metal sheet is positioned over the drawing mold and then laid onto the curved or contoured contact surface of the drawing mold. The edges or side margins of the metal sheet located opposite each other in a horizontal plane are secured to the tensile drawing apparatus, which then applies a pre-stressing tension during a defined pre-stressing time on the contoured contact surface of the drawing mold, under the effect of room temperature and normal ambient atmospheric pressure (e.g. without supplying heated pressurized air through the air holes of the mold surface). Thereby, the metal sheet is drawn and deformed from its initial neutral condition into a pre-drawn or pre-bent state by being stretched along a pre-stressing deformation displacement distance.
Next, tempered or heated pressurized air at a constant pre-heating air pressure is introduced through the air holes of the drawing mold, with an increasing temperature until it reaches a heating temperature after the course of a heating time. Thereby, the pressurized air spreads out between the metal sheet contact surface and the drawing mold contact surface to form an air cushion therebetween. Also, during this heating phase, the metal sheet, which is under the above mentioned pre-stress, will be heated up to the above mentioned heating temperature of the heating air, although the drawing tension will not be changed so there will be no further deformation and no change of the pre-drawn or pre-bent condition.
Then, in a subsequent deforming phase, an increased constant deforming air pressure is applied by means of the pressurized air through the air holes so that the metal sheet is supported in a floating manner on the resulting air cushion with essentially no friction relative to the contoured contact surface of the drawing mold. Also during this phase, the temperature of the metal sheet is held constant at the above mentioned heating temperature, and the tensile drawing machine applies an additional drawing strain so that the metal sheet is formed to have a curvature or contour fitting the surface contour of the contact surface of the drawing mold, as the metal sheet is iteratively or stepwise or constantly stretched or drawn further through a forming displacement distance.
Thereafter, in a cooling phase, the pressurized air is cooled down from the heating temperature to room temperature, at a constant cooling air pressure with which it is provided through the air holes of the drawing mold. During this phase, the metal sheet is also cooled down, while maintaining the formed contour that was established in the prior step or phase.
Next, an optional calibrating step may be carried out, whereby the metal sheet, which has been formed through the prior steps, is subjected to a further drawing tension and strain at atmospheric pressure and room temperature, to bring the final curvature or contour of the formed metal sheet to an exact required dimension or configuration.
The method encompasses variations of the above process, and further details of the various steps. For example, the edges of the metal sheet could first be secured to the tensile drawing machine, and thereafter moved downwardly onto the drawing mold, to move and pull the sheet onto the curved mold surface.